Research of Heat Storage Tank Operation Modes in Cogeneration Plant ; Kogeneracinės jėgainės šilumos akumuliacinės talpos veikimo režimų tyrimai
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VILNIUS GEDIMINAS TECHNICAL UNIVERSITY Giedrė STRECKIENĖ RESEARCH OF HEAT STORAGE TANK OPERATION MODES IN COGENERATION PLANT SUMMARY OF DOCTORAL DISSERTATION TECHNOLOGICAL SCIENCES, ENERGETICS AND POWER ENGINEERING (06T) Vilnius 2011 Doctoral dissertation was prepared at Vilnius Gediminas Technical University in 2006–2011. Scientific Supervisor Prof Dr Habil Vytautas MARTINAITIS (Vilnius Gediminas Technical University, Technological Sciences, Energetics and Power Engineering – 06T). Consultant Prof Dr Habil Petras VAITIEKŪNAS (Vilnius Gediminas Technical Uni-versity, Technological Sciences, Environmental Engineering – 04T). The dissertation is being defended at the Council of Scientific Field of Energetics and Power Engineering at Vilnius Gediminas Technical University: Chairman Prof Dr Habil Rimantas KAČIANAUSKAS (Vilnius Gediminas Technical University, Technological Sciences, Energetics and Power Engineering – 06T). Members: Prof Dr Egidijus Saulius JUODIS (Vilnius Gediminas Technical Univer-sity, Technological Sciences, Energetics and Power Engineering – 06T), Assoc Prof Dr Arnas KAČENIAUSKAS (Vilnius Gediminas Technical University, Technological Sciences, Mechanical Engineering – 09T), Dr Robertas POŠKAS (Lithuanian Energy Institute, Technological Sciences, Energetics and Power Engineering – 06T), Prof Dr Habil Stasys ŠINKŪNAS (Kaunas University of Technology, Technological Sciences, Energetics and Power Engineering – 06T).
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| Publié par | vilnius_gediminas_technical_university |
| Publié le | 01 janvier 2011 |
| Nombre de lectures | 32 |
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VILNIUS GEDIMINAS TECHNICAL UNIVERSITY
GiedrSTRECKIEN
RESEARCH OF HEAT STORAGE TANK OPERATION MODES IN COGENERATION PLANT
SUMMARY OF DOCTORAL DISSERTATION
TECHNOLOGICAL SCIENCES, ENERGETICS AND POWER ENGINEERING (06T)
Vilnius
2011
Doctoral dissertation was prepared at Vilnius Gediminas Technical University in 2006–2011 . Scientific Supervisor Prof Dr Habil Vytautas MARTINAITIS Gediminas Technical (Vilnius University, Technological Sciences, Energetics and Power Engineering – 06T).ConsultantProf Dr Habil Petras VAITIEKNAS(Vilnius Gediminas Technical Uni-versity, Technological Sciences, Environmental Engineering – 04T). The dissertation is being defended at the Council o f Scientific Field of Energetics and Power Engineering at Vilnius Gediminas Technical University: ChairmanProf Dr Habil Rimantas KASAKSUAANI(Vilnius Gediminas Technical University, Technological Sciences, Energetics and Power Engineering – 06T).Members:Prof Dr Egidijus Saulius JUODIS (Vilnius Gediminas Technical Univer-sity, Technological Sciences, Energetics and Power Engineering – 06T), Assoc Prof DrArnas KAKSUASAENI(Vilnius Gediminas Technical University, Technological Sciences, Mechanical Engineering – 09T), Dr Robertas POŠKAS(Lithuanian Energy Institute, Technological Sciences, Energetics and Power Engineering – 06T), Prof Dr Habil Stasys ŠINKNAS University of Technology, (Kaunas Technological Sciences, Energetics and Power Engineering – 06T). Opponents:Prof Dr Habil BenediktasSNA (Vilnius Gediminas Technical Univer-sity, Technological Sciences, Energetics and Power Engineering – 06T), Prof Dr Habil Gintautas MILIAUSKAS University of Techno- (Kaunas logy, Technological Sciences, Energetics and Power Engineering – 06T). The dissertation will be defended at the public meeting of the Council of Scientific Field of Energetics and Power Engineering in the Senate Hall of Vilnius Gediminas Technical University at 2 p. m. on 9 June 2011. Address: Saultekio al. 11, LT-10223 Vilnius, Lithuania. Tel.: +370 5 274 4952, +370 5 274 4956; fax +370 5 270 0112; e-mail: doktor@vgtu.lt The summary of the doctoral dissertation was distributed on 6 May 2011. A copy of the doctoral dissertation is available for review at the Library of Vilnius Gediminas Technical University (Saultekio al. 14, LT-10223 Vilnius, Lithuania). © GiedrStreckien, 2011
VILNIAUS GEDIMINO TECHNIKOS UNIVERSITETAS
GiedrSTRECKIEN
KOGENERACINS JGAINS ŠILUMOS AKUMULIACINS TALPOS VEIKIMO REŽIMTYRIMAI
DAKTARO DISERTACIJOS SANTRAUKA
TECHNOLOGIJOS MOKSLAI, ENERGETIKA IR TERMOINŽINERIJA (06T)
Vilnius
2011
Disertacijarengta2006–2011metaisVilniausGediminotechnikos universitete. Mokslinis vadovas prof. habil. dr. Vytautas MARTINAITIS Gedimino technikos (Vilniaus universitetas, technologijos mokslai, energetika ir termoinžinerija – 06T). Konsultantas prof. habil. dr. Petras VAITIEKNAS(Vilniaus Gedimino technikos universitetas, technologijos mokslai, aplinkos inžinerija ir kraštotvarka – 04T).Disertacija ginama Vilniaus Gedimino technikos universiteto Energetikos ir termoinžinerijos mokslo krypties taryboje: Pirmininkas prof. habil. dr. Rimantas KAIANAUSKAS Gedimino (Vilniaus technikos universitetas, technologijos mokslai, energetika ir termoinžine-rija – 06T). Nariai: prof. dr. Egidijus Saulius JUODIS Gedimino technikos (Vilniaus universitetas, technologijos mokslai, energetika ir termoinžinerija – 06T), doc. dr. Arnas KAENIAUSKAS(Vilniaus Gedimino technikos universitetas, technologijos mokslai, mechanikos inžinerija – 09T), dr. Robertas POŠKAS (Lietuvos energetikos institutas, technologijos mokslai, energetika ir termoinžinerija – 06T), prof. habil. dr. Stasys ŠINKNAS technologijos universitetas, (Kauno technologijos mokslai, energetika ir termoinžinerija – 06T). Oponentai: prof. habil. dr. BenediktasSNA Gedimino technikos (Vilniaus universitetas, technologijos mokslai, energetika ir termoinžinerija – 06T), prof. habil. dr. Gintautas MILIAUSKAS (Kauno technologijos universitetas, technologijos mokslai, energetika ir termoinžinerija – 06T). Disertacija bus ginama viešame Energetikos ir termoinžinerijos mokslo krypties tarybos pos technikos odyje 2011 m. birželio 9 d. 14 val. Vilniaus Gedimin universiteto senato posdžisalje. Adresas: Saultekio al. 11, LT-10223 Vilnius, Lietuva. Tel.: (8 5) 274 4952, (8 5) 274 4956; faksas (8 5) 270 0112; el. paštas doktor@vgtu.lt Disertacijos santrauka išsiuntinta 2011 m. gegužs 6 d. Disertacijgalima peržirti Vilniaus Gedimino technikos universiteto bibliotekoje (Saultekio al. 14, LT-10223 Vilnius, Lietuva). VGTU leidyklos „Technika“ 1875-M mokslo literatros knyga. © GiedrStreckien, 2011
Introduction Topicality of the problem. Solution of heat accumulation is a wide field for scientific research, bridging both theoretical scientific issues of heat and mass transfer and practical applications. Since 1970 the research on heat and mass transfer in the heat storage tanks was intensively undertaken in order to maintain and regain the high quality heat from the storage tank. However, the developing modern technologies allow analysing processes in the storage tank in a more rapid, cheaper and more accurate way. Combination of heat accumulation and cogeneration (CHP) technologies assists in achievement of goals of energy production efficiency and environmental pollution reduction. However, the potential of optimisation possibilities of small-scale CHP plants has not availed, and recent studies of CHP plants with heat storage and the created optimisation methods are usually narrowed by only economical selection of the equipment size. The main problem of these studies to be solved converges to a finding of minimal maintenance and operation expenses of the CHP plant under certain economic conditions. In these cases the thermal processes inside the storage tank are not examined, and this equipment in the system may be referred as “a black box”. Research of operation modes of heat storage tank in the CHP plant would enable to use the theoretical knowledge in practice, contributing to a more precise selection of storage tank volume and results of modelling of the entire energy production system. It should be noted, that selection problem of CHP plant with heat storage is topical not only in Lithuania, but also in other countries. Object of research– operation modes of heat storage tank of small-scale CHP plant and thermal stratification that formed during its operation. Aim and tasks of the work. aim of this work is to investigate The peculiarities of operation modes of heat storage tank in small-scale CHP plant, develop an algorithm allowing to choose the tank volume and present a model allowing determination of thermal stratification in the storage tank at any time of its operation. The tasks of the work are: 1.To investigate the characteristic operation modes of heat storage tank in small-scale CHP plant. To evaluate the impact of consumers’ demands changes and electricity tariffs on the operation of the CHP plant with heat storage tank.
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2.Having determined the key factors having impact on the heat storage tank size and operation, to develop the algorithm for selection of economically optimal tank volume for small-scale CHP plant. 3.the heat and mass transfer processes, taking process inHaving assessed the heat storage tank, to form the semi-analytical and numerical models, which allow determining thermal stratification at any moment of operation of the heat storage tank. 4.To compare the results of numerical and semi-analytical modelling with the actual data of the heat storage tank. 5.To present the recommendations for engineering calculations in selecting of heat storage tank for the CHP plant and determining thermal stratification in the storage tank. Methodology of research. order to perform the defined tasks, the In combination of various methods and models is applied in the work. A case study method, implemented on the base of simulation modelenergyPRO, and economical research methodology are chosen for technical and economical evaluation of the heat storage tank in the CHP plant. Sensitivity analysis is employed for risk assessment. The semi-analytical and numerical research, which uses the packagePHOENICSincorporating the finite volume method, is applied in the analysis of thermal processes in the storage tank. Scientific novelty1.Analysis of selection of the storage tank volume for small-scale CHP plant is performed with a reference to consumers’ demand, electricity tariffs and CHP plant operation strategy. Investigated cases of CHP plant installation are: –when this system satisfies a part of consumer’s electricity demand and –when all electricity produced by CHP unit is sold in spot market. 2.Numerical models (two-dimensional and three-dimensional) are created and two semi-analytical models are adapted for simulating the thermal stratification in heat storage tanks installed both in CHP plants and other energy production systems. Practical value. of heat storage tank, as presented in this Assessment work, can serve a versatile analysis of CHP plants. The research results may be used during planning and engineering design of CHP plants, when the optimal storage tank volume is determined in the system adequately to the consumption. The created semi-analytical models may be easily integrated to optimisation and simulation models enabling selection and analysis of the energy production
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systems with heat storages. This would allow estimating thermal processes and thermal stratification in the storage tank. Meanwhile, numerical model can be applied for more specific research of stratified storage tanks. Defended propositions1.When the CHP unit in the CHP plant operates at nominal power, only two characteristic operation regimes are formed in heat storage tank. 2.When the curves of consumers’ energy demand are more complicated, more potential alternatives occur for operation strategy of CHP unit and heat storage tank. 3.Greater fluctuation in electricity prices in spot market promotes the use of CHP plants with heat storage. 4.The created numerical model, the prepared semi-analytical energy balance and “plug flow” models are proper to be used in research of heat storage tanks of different volume and purpose. 5.the created numerical model are moreApplication possibilities of flexible and more diverse than of semi-analytical models, however, this model is more computationally expensive and requires investigator’s specific technical skills. The scope of the scientific work. scientific work consists of the The general characteristic of the dissertation, 4 chapters, general conclusions, list of literature, list of publications and addenda. The total scope of the dissertation is 136 pages, 65 pictures, 11 tables and 2 addenda. 1. Analysis of heat accumulation in storage tanks In this chapter the possibility of heat accumulation in the stratified thermal energy storage tanks is analysed. The literature review has showed that the formation of the thermal stratification is determined by the geometry of the storage tank, the inlet, the hydrodynamics and thermal characteristics of the water flow in the tank. With advancement of computer technologies, more two-dimensional and three-dimensional simulations are performed in thermal stratification field. However, there is still noticed the demand of numerical and analytical models that can furnish the results in an accurate and rapid way. One of the fields of thermal energy storage application is the installation of heat storage tank in CHP plant. In this system the heat storage tank provides flexibility for CHP plant, as plant operation becomes less dependent on consumers’ heat demand that is alternating in time. With storage tank installed, the production of electricity and heat can be uncoupled for a period of time. Such uncoupling can be very beneficial for enabling maximum electricity
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production during the hours where electricity is being paid the best. Studies that were performed in CHP with heat storage usually approach only economical aspects of the CHP plant. In the accomplished engineering calculations and optimisation models the heat storage tanks are examined as non-stratified tanks. Considering this, there appear a need to integrate into the available software the unsophisticated engineering calculations and analytical methods that are designed to determine the thermal stratification in the heat storage tank. 2. Methods for selecting storage tank size and determining thermal stratification In this chapter an economical model of selection of storage tank volume is described. The model consists of consumers’ demand segregation, evaluation of economic and technological circumstances, composition of cases to be analysed, introduction of principal scheme of the analysed system, finding of an algorithm of economically optimal storage tank volume. A simple CHP system is investigated. The system contains a CHP unit (gas engine), a peak boiler and a hot water storage tank.energyPRO software has been chosen for the economic analysis because it allows the user to carry out a comprehensive, integrated and detailed technical analysis. The economic feasibility analysis of the chosen CHP plant configuration is based on the net present value (NPV) and simple payback time. The NPV is selected as the preferred criteria for the optimality of the CHP plant configuration. Two models are prepared in order to carry out the semi-analytical determination of thermal stratification in the heat storage tank: energy balance model and “plug flow” model. During analytical investigation of temperature distribution in the heat storage tank at any time of its operation, the entire tank is divided intoN equal layers. In energy balance model an every layer of this storage tank is described by following equation: (micp)ddTiipmcppTpTiidmcpdTiTdkApavTiTa (1) imicpTi1Tiimi1cpTiTi1AeffiTi12TiTi1, zi wherem – mass;cp – specific heat capacity at constant pressure;T – temperature; – time;k– overall heat transfer coefficient;Apav– exterior surface of the layer;Ai – cross-section are of the layer;eff – effective vertical
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heat conductivity;zi– height of the layer;– parameter which indicates if flow exist or not; subscripts:p– production;d– demand. In “plug flow” model the volume of inflow during the particular period is inserted in to the model of the tank, and correspondingly the same fluid volume is removed. Layers in the tank are changed from above downwards or from bottom upwards with reference to the nature of the process, i.e. charging or discharging. Implementing this model every layer of this tank is described by following equation: kA T T T Texp , i,a iamipcipav (2) , whereTi,– temperature of the respective layer after time. In semi-analytical calculations the mass of layeriis assessed to be product of water density and volume. As the water density and specific heat depends on temperature, so these values are assessed to be functions subordinate to temperature. Numerical modelling of thermal stratification in heat storage tank is performed usingPHOENICS software, based on a finite volume method. Transient heat and mass transfer processes bound in the storage tank are described by continuity, momentum and energy equations. In order to check the accuracy of results received using different models, the data of the real heat storage tank inHvide SandeCHP plant are used. This storage tank is 14.99 m height, 12.9 m in diameter; it has 0.3 m of thermal insulation. During operation of CHP plant, temperature of hot water fed to the top of the tank reaches about 94–95 ºC temperature of cold water fed from the , bottom of the tank is less, about 42–45 ºC. Temperature data of the tank are obtained from 15 PT100 temperature sensors. Lowest temperature sensor is installed in height of 0.5 m above the bottom of the tank; the highest sensor is located 0.5 m below upper diffuser, which is fitted in height of 14.49 m. Error of the used temperature sensors ranges from ±0.50 ºC to ±0.78 ºC when the temperature of stored water is in range 40 ºC to 95 ºC. 3. Economic analysis of storage tank volume selection Performing search and analysis of economically optimal tank volume in CHP plant, several different consumers’ demands were analysed. TypeAconsumer is characterised by the scenario, when there is only one increase in demand of domestic hot water (DHW) and electricity during one day, and type
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B when there are two increases in demand of DHW and electricity. The – examined annual consumers’ demands varied in the following ranges: demand of electricity 500–5000 MWh; heat for heating of premises – 1200–14500 MWh and heat for DHW preparation – 300–3500 MWh. During analysis it was determined that when CHP unit has priority to operate all day, and the generated electricity is possible to use not only for the own needs, but also for sale, then in cases of typeA consumer it is most economically beneficial to install the CHP unit satisfying the greatest consumer’s demand for electricity. Such a CHP unit provides 32–44 % of necessary annual heat. Economically optimal relative tank volume of 10– 17 m3/1000 MWh of annual heat delivered from CHP unit is required. In cases of typeBconsumers it is received that CHP unit of optimal power with optimal tank volume provides about 33–46 % of required annual heat. After calculations it is determined that optimal relative tank volume of 7– 9 m3 plants, when the/1000 MWh of delivered annual heat is required in CHP tank is charged twice per day, and CHP unit operates only at peak periods. If it is supposed that CHP unit would have only one start-up per day, then the relative tank volume of 14–19 m3/1000 MWh of delivered annual heat is required. After study of consumer, whose annual demands are following: 2000 MWh for electricity, 1800 MWh for DHW and 5500 MWh for heating, it is determined that at single time interval electricity tariff the economically optimal st3 C
130 kWe 190 kWe 250 kWe 320 kWe 400 kWe
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 10 20 30 40 50 60 70 Storage volume, m3 Fig. 1.NPV dependency given a different combination of CHP plant equipment, typeB
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Operation of tanks of different size in the CHP of 320 kWe non- during heating period, in case of typeBconsumer, is represented in Fig. 2. As it can be seen, the tank, which volume is small (5 m3 and 15 m3), often is charged and discharged during one day, and simultaneously it interferes the operation of CHP unit, which is to be additionally stopped and again re-started. At the same time, the tank of too big volume can transpose the production of CHP unit in time, if in the strategy of CHP plant operation it is intended that CHP unit operates until the time when the tank is fully charged and it does not operate at those periods when the storage tank satisfies all the demands of consumers for DHW. Thus, the highest economic impact is achieved not in all cases of increasing the volume of heat storage tank. 2.5 2 1.5 1 0.5 0 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 Time 3 3 3 5 m 15 m 17 m 25 m335 m350 m3 Fig. 2.Operation of tanks of different size in the CHP of 320 kWe, when CHP unit has priority to operate all day, typeBInvestigating the strategy of CHP plant operation, when the CHP unit has priority to operate only at the day-time tariff, it was determined that in cases of typeA given a possibility to sell the electricity, the CHP unit of consumers, optimal power provides about 20–25 % of required annual heat, and optimal relative tank volume of 40–47 m3/1000 MWh of annual heat delivered from CHP unit is required. In cases of no possibility to sell the electricity, it was obtained that optimal relative volume decreased and formed 18– 30 m3/1000 MWh annual heat delivered from CHP unit. In cases of typeB consumers, given a possibility to sell the electricity, it was obtained that it was most economically beneficial to install the CHP unit that would assist to satisfy the maximum electricity demands. In case of this selection of CHP unit, it provides 21–29 % of required annual heat. Under these
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